FIG. 7 shows the configuration of a usual power device for a gas laser oscillator disclosed in JP-A-9-129953. The AC voltage of a commercial power source 1 is converted to a DC voltage in a converter part 2 and inputted to an inverter part 3. In the inverter part 3, switching elements are turned on and off by a gate signal of a gate signal output circuit 10 and the DC voltage is converted to a square wave AC voltage. The output voltage of the inverter part 3 is boosted by a high frequency transformer 4 having an inductance L and applied to a part between dielectric electrodes 5a and 5b having an electrostatic capacity C, so that a discharge 6 is generated. A discharge current supplied between the dielectric electrodes 5a and 5b has its quantity set by a command value outputted from an NC device 9. The gate signal output circuit 10 outputs the gate signal to the inverter part 3 under a discharge frequency fso (>½π√{square root over ( )} (LC)) by a PWM control on the basis of the output value of a discharge current detecting circuit 8 for detecting the discharge current and the command value of the NC device 9.
The operation of the inverter part 3 carried out when the discharge 6 is generated (refer it to as during discharge on period, hereinafter) is described below. FIG. 8 shows the configuration of the inverter part 3 in the related art. The inverter part 3 includes switching elements 11a, 11b, 11c and 11d and circulating current diodes 12a, 12b, 12c and 12d connected in parallel with them. FIG. 9 shows examples of the wave forms of an output voltage and a current of the inverter part 3 when the inverter part 3 is controlled in accordance with the PWM system. The plus of the wave form of the voltage and the wave form of the current in FIG. 9 indicate that in the wave form of the voltage, an arrow mark of the output voltage is directed toward a plus (a high potential) side in FIG. 8 and, in the wave form of the current, the output current flows in the direction of an arrow mark in FIG. 8. At this time, to highly efficiently reduce the switching loss of the elements forming the inverter part 3, the inverter part 3 ordinarily operates under the frequency fso (>½π√{square root over ( )} (LC)) slightly higher than a series resonance frequency fr (=½π√{square root over ( )} (LC)) determined by the inductance L of the high frequency transformer 4 and the electrostatic capacity C of the dielectric electrodes 5a and 5b. As shown in FIG. 9, the output current of the inverter part 3 has a phase lag relative to the output voltage of the inverter part. When the switching element 11b is turned off from a state that the switching elements 11a and 11b are turned on (during t1 shown in FIG. 9), a circulating current If shown by a broken line in FIG. 8 is supplied to the circulating current diode 12c in the direction of the plus (during t2 shown in FIG. 9). Under this state, the switching element 11c is turned on, and then, the switching element 11d is turned on. When the switching element 11d is turned on, that is, at a point B in FIG. 9, a backward voltage is applied to the circulating current diode 12a, however, in the point B, since the circulating current If flows in the direction of the plus, a forward current is not supplied to the circulating current diode 12a. Accordingly, in the circulating current diode 12a, a recovery current is not generated.
Similarly, when the switching element 11d is turned off from a state that the switching elements 11c and 11d are turned on (during t3 shown in FIG. 9), a circulating current If′ is supplied to the circulating current diode 12a in the direction opposite to that of the circulating current If shown by the broken line in FIG. 8, that is, in the direction of the minus (during t4 as shown in FIG. 9). Under this state, the switching element 11a is turned on, and then, the switching element 11b is turned on. When the switching element 11b is turned on, that is, at a point A shown in FIG. 9, a backward voltage is applied to the circulating current diode 12c, however, at the point A, the circulating current If′ flows in the minus direction, a forward current is not supplied to the circulating current diode 12c. Accordingly, a recovery current is not generated in the circulating current diode 12c. 
As described above, in the related art, the output current ordinarily has the phase lag relative to the output voltage, so that when the discharge is generated, the recovery current is adapted not to be generated in the circulating current diode.
However, when the discharge 6 is not generated (refer it to as during discharge off period, hereinafter), the discharge current is not continuously supplied until the discharge is generated by applying the voltage to the dielectric electrodes, though the voltage is applied. The wave forms of the output voltage and the current of the inverter part 3 in this case are, for instance, shown in FIG. 10. The plus of the voltage wave form and the current wave form shown in FIG. 10 indicates that in the wave form of the voltage, an arrow mark of the output voltage is directed toward a plus (a high potential) side in FIG. 8, and in the wave form of the current, the output current flows in the direction of an arrow mark in FIG. 8. As shown in FIG. 10, the output current of the inverter part 3 during discharge off period has a wave form asynchronous with the wave form of the output voltage. This phenomenon arises because of a reason why when the discharge is generated between the dielectric electrodes 5a and 5b, a gap between the dielectric electrodes 5a and 5b serves as a DC resistance component, however, when the discharge is not generated, the gap serves as a capacitance, the circuit is equivalent to a circuit in which the capacitance is inserted in series to the dielectric electrodes 5a and 5b. Thus, since the impedance and the resonance frequency of the circuit change, a dark current having a peak or a frequency determined by them flows. As described above, since the circuit is equivalent to the circuit in which the capacitance is inserted in series to the dielectric electrodes 5a and 5b, the electrostatic capacity of the entire part of the discharge part is ordinarily decreased and the resonance frequency rises.
As shown in FIG. 10, when the dark current flows, at a point A when the switching element 11b in FIG. 10 is turned on, the switching element 11b is turned on and the backward voltage is applied to the circulating current diode 12c. However, at the point A, the dark current is supplied in the direction of the plus, namely, from the switching element 11a to the high frequency transformer and to the circulating current diode 12c. Since the forward current is supplied to the circulating current diode 12c, the recovery current is supplied to the circulating current diode 12c. Accordingly, an abnormal heat generation arises in the circulating current diode 12c. 
Similarly, at a point B when the switching element 11d in FIG. 10 is turned on, the switching element 11d is turned on and the backward voltage is applied to the circulating current diode 12a. However, at the point B, the dark current is supplied in the direction of the minus, that is, from the switching element 11c to the high frequency transformer and to the circulating current diode 12a. Since the forward current is supplied to the circulating current diode 12a, the recovery current is supplied to the circulating current diode 12a. Thus, an abnormal heat generation arises in the circulating current diode 12a. 
As described above, in the power device for the gas laser oscillator having a load discontinuous between during discharge on period and during discharge off period, to meet the above-described problem generated during discharge off period according to the related art, the number of the circulating current diodes connected in parallel can be merely increased to distribute the loss of the circulating current diode due to the recovery current. Meanwhile, in recent years, a discharge frequency in the power device for the gas laser oscillator becomes progressively high and the response speed of an employed circulating current diode is requested to be high. Thus, the loss due to the recovery current is also increased. Accordingly, many relatively expensive and high speed diodes need to be used in parallel, which results in a very serious problem in view of the rise of a cost and the increase of a mounting space.